Is this really a big factor?
Not being multiplanetary seems like the least of our existential problems here on Earth, and will continue to be that way for awhile.
At the same time, chemical rocket efficiency becomes totally irrelevant for a slightly more advanced civilization than us.
You can't build a space elevator before getting to the orbit first.
A jet engine capable of leaving a deep gravitational well must have a big ratio of thrust to weight. If a chemical rocket is too weak, a nuclear jet engine is the only remaining option. Would you be comfortable running it in the thick atmosphere of a densely inhabited planet?
A civilization just a few decades ahead of us is (theoretically) almost unimaginable. Bio augmentation, true AIs, who knows what advances in fundamental physics knowledge... Just to start.
What I'm saying is that whatever engineering and environmental limitations we currently perceive are probably irrelevant.
> Not being multiplanetary seems like the least of our existential problems here on Earth, and will continue to be that way for awhile.
That might be true, especially if your 'for awhile' talks about millennia at most.
But it's an extremely relevant concern in the context of the Fermi paradox.